How often do you lend books from the library? If your answer is on a real regular basis, get a Nook. Is it is rarely, go with the Kindle.
The Kindle will have the best screen on the market (at least for the time being), better PDF usability then other readers (annotation, dictionary, search, landscape), a huge harddrive (4 GB), and is one of the rare international ereaders out there (you can use it in many countries outside the US). And it is less expensive and it has a larger bookstore.
To Kill a Mockingbird is not approved for e-book release and since it is under copyright protection it has to be released by the author, which she has not done. The publisher who owns the paper rights would say that permission is needed from them but the courts have ruled that paper rights do not extend to e-book rights in contracts where there was no indication of e-readers. The Publishers are fighting this ruling but for now, Harper Lee would have to allow it to be made into an ebook.